This dissertation study explores a young Japanese heritage language speaker's learning process of formal writing in Japanese. With a rapid increase in immigrants into the U.S. and growing importance of diversity, many researchers have shown interest in immigrant children, i.e., heritage speakers, from educational and research perspectives and have found that many heritage speakers, even though they have a native-like fluency in informal conversations, cannot read and write in their heritage languages as native speakers at their age do. Regarding Japanese heritage speakers, Japanese supplementary schools play an important role in fostering their Japanese proficiency and cultural identities, but once children leave supplementary schools around age nine because of difficulty in curricula, it is hard for them to acquire advanced Japanese afterward. This dissertation provides a four-week intensive academic Japanese reading and writing program based on a usage-based language acquisition for the Japanese heritage speaker, Sakura, who has never attended Japanese schools. Her argument essays drastically improve, which empirically supports the efficacy of usage-based instruction for formal writing to some degree. The qualitative analysis of this study and its findings suggest multiple implications and possibilities for future research: (1) it is generally held that kango 'Sino-Japanese words' are more challenging for learners than "wago" 'Japanese native words,' but the most difficult words for Sakura are some extremely topic- and genre-specific "wago"; (2) new words are registered in her receptive lexicon through their frequent occurrences, but advancing them to productive lexicon necessitates varying contexts, collocations, and functions; (3) L1 transfer is not limited to beginners, but Sakura's writing also reveals an influence from English to satisfy her L1 linguistic sophistication; and (4) the most crucial factor in Sakura's improvement is her strong "mission-like" motivation with a sense of responsibility, which I call heritage motivation and which is not extractable from a conventional quantitative approach. This motivation has grown in her mind through a positive attitude, or somatic value, toward Japanese and their culture. It suggests a focal shift from "how teachers can teach learners" to "how teachers and others can nurture their positive somatic value leading to motivation." [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]